1. Field of the invention
The present invention relates to a process for efficiently producing a vinyl chloride polymer with a high degree of polymerization which affords molded products little smelling and less liable to initial discoloration.
2. Description of the prior art
A vinyl chloride polymer is usually produced by stirring and suspending vinyl chloride or a vinyl monomer mixture containing vinyl chloride in an aqueous medium and adding to the suspension an oil-soluble polymerization initiator. Production of vinyl chloride polymers in this manner takes a long polymerization time, and attempts are being made to reduce it for improvement in productivity. One way to achieve this object is to increase the amount of catalyst (polymerization initiator) to be added. Unfortunately, the result is that the vinyl chloride polymer affords molded products smelling strongly and liable to initial discoloration.
Ordinary suspension polymerization of vinyl chloride monomer employs one or more oil-soluble polymerization initiators which have a half-life period of 10 hours at 30 to 50.degree. C. in benzene (0.05 mol/L). After polymerization reaction in this manner, the oil-soluble polymerization initiators leave unreacted components and reaction residues which are adsorbed by the polymer. They are decomposed by heat while the polymer undergoes thermal processing. The result is a deteriorated color. This is the reason why oil-soluble polymerization initiators adversely affect initial discoloration if they are used in large amounts. oil-soluble polymerization initiator, wherein the improvement comprises using a diacylperoxide compound and t-hexylperoxy diglycolate in combination with each other as the oil-soluble polymerization initiator, said diacylperoxide compound having a half-life period of 10 hours at 30 to 50.degree. C. in benzene (0.05 mol/L). The resulting vinyl chloride polymer has a high degree of polymerization and affords high-quality molded products little smelling and less liable to initial discoloration.
The above-specified oil-soluble polymerization initiator offers the following advantages. It does not contain the neodecanate group which is a source of odor. It leaves residues, but they easily vaporize under ordinary drying conditions owing to their low boiling point and hence do not remain in the polymer. It remains highly active during polymerization and hence permits polymerization in a comparatively short time even though its amount is reduced. This favorably affects the initial discoloration of molded products. It permits a vinyl chloride resin having a high degree of polymerization (higher than 1500, especially higher than 2000) to be produced efficiently in a short time.
The present inventors further found that the oil-soluble polymerization initiator exhibits a higher activity when the above-mentioned combination of diacylperoxide and t-hexylperoxy diglycolate is further combined with t-butylperoxy neooctanoate, so that it is possible to reduce the amount of the first two compounds. The effect is that it is possible to raise the temperature of the cooling water passing through the jacket during polymerization. This prevents odors and contributes to the efficient production of vinyl chloride polymers. These findings led to the present invention.
Thus, the present invention provides an improved process for producing a vinyl chloride polymer by suspension polymerization of vinyl chloride monomer in an aqueous medium in the presence of an oil-soluble polymerization initiator, wherein said improvement comprises using a diacylperoxide compound and t-hexylperoxy diglycolate in combination with each other as the oil-soluble polymerization initiator, said diacylperoxide compound having a half-life period of 10 hours at 30 to 50.degree. C. in benzene (0.05 mol/L). The present invention also provides the process for producing vinyl chloride polymer as defined above, wherein the combination of diacylperoxide and t-hexylperoxy diglycolate is further combined with t-butylperoxy neooctanoate.